posted by Heena .
Which of the following substances has a ∆H°f equal to zero?
All elements are zero. A few elements (carbon, tin and a few others) occur as allotropes; therefore, some arbitrary decisions have been made about which one will be called the "standard." In the case carbon (diamonds, graphite, amorphous carbon, nanofoam, etc) graphite is the one. In the case of Sn, it is the white form. Other elements are in this list for you to name.